This application claims the benefit of priority under 35 U.S.C. 119 to Russian patent application no. 2000117336, filed Jul. 4, 2000, which is hereby incorporatd by reference.
1. Field of the Invention
The invention relates to a method and device for producing extremely short-wave UV and soft X-ray radiation from a dense hot plasma discharge of pinch type. The field of application includes lithography, particularly in the spectral range around 13.5 nm, lasers in the short-wave UV and X-ray ranges, and X-ray microscopy.
2. Discussion of the Related Art
A method is known for producing short-wave radiation at xcex=13.5 nm using a plasma focus (see U.S. Pat. No. 5,763,930, hereby incorporated by reference). However, a condition of effective operation is the addition of lithium vapor to the inert gas contained in the discharge chamber, and this substantially complicates the design of the source of radiation and contaminates the space outside the discharge.
A method of producing short-wave radiation with the aid of a z-pinch involving RF pre-ionization is devoid of this disadvantage, but the dielectric wall of the discharge chamber at which the pinch-type discharge is initiated is subject both to exposure to powerful radiation flux and the substance that forms as a result of electrode erosion (see U.S. Pat. No. 5,504,795, hereby incorporated by reference). This limits the possibilities of achieving a long service life when this approach is implemented.
A close technical achievement is a method of producing short-wave radiation from a gas-discharge plasma that consists in the pre-ionization of gas in the discharge region between coaxial electrodes achieved through an axial aperture in one of the electrodes, and in initiating a pinch-type discharge (see German patent DE 197 53 696 A1, hereby incorporated by reference).
The device for implementing this method contains a discharge chamber having two axially symmetrical electrodes optically communicating through an aperture formed in one of the electrodes, with a source of pre-ionization disposed outside the discharge chamber (see the ""696 published application).
In this method and device, pre-ionization is achieved by a low-current discharge that is automatically formed in a cavity of the cathode when discharge voltage is applied and that then propagates into the discharge gap through the aperture in the hollow cathode. The internal dielectric wall of the discharge chamber may be disposed outside the zone irradiated by the discharge, and this enables a long service life to be achieved in a periodically pulsed operating mode.
Disadvantages of this method and the device for implementing it are a low efficiency of conversion of the energy input into radiation in the short-wave range due to the low level of pre-ionization and its non-ideal spatial distribution in the gap between the electrodes of the discharge chamber. Since the pre-ionization is carried out substantially in the paraxial region of the discharge gap, increasing the cross-sectional area of a pinch-type discharge is made difficult at its initial stage, and this limits the possibility of increasing the energy and the average power of the short-wave radiation. In addition, the long time of formation (approximately 1 ms) of the automatic pre-ionization and of the initiation of a pinch-type discharge compared with the time interval between individual pulses and the low rate of growth (approximately 107 V/s) of the discharge voltage limit the possibility of achieving a high radiation energy stability from pulse to pulse.
It is desired to provide an increase in the efficiency, average power and stability of short-wave radiation of a gas-discharge plasma.
In accordance with this object, a method is provided for producing short-wave radiation from a gas-discharge plasma, including pre-ionization of the gas in the discharge region between coaxial electrodes achieved through an axial aperture formed in one of the electrodes and initiation of a pinch-type discharge. Pre-ionization is achieved simultaneously by a flux of radiation having wavelengths from the UV to X-ray range and by the flux of accelerated electrons from the plasma of the pulsed sliding discharge initiated in a region not optically communicating with the axis of the pinch-type discharge. A rate of growth of the discharge voltage across the region preferably and advantageously exceeds 1011 V/s. Fluxes of radiation and electrons are preferably formed axially symmetrically and are directed into part of the discharge region outside the axis.
The method can be implemented by a device containing a discharge chamber having two axially symmetrical electrodes optically communicating through an aperture formed in one of the electrodes, with a source of pre-ionization disposed outside the discharge chamber. The source of pre-ionization preferably derives from an axially symmetrical system of forming a sliding discharge comprising an elongated initiating electrode coated with a dielectric layer, on the surface of which there is disposed a trigger electrode, the initiating electrode being arranged coaxially with the electrodes of the discharge chamber and formed so that the dielectric layer is disposed in a region not optically communicating with the axis of the discharge chamber and one of the electrodes of the system for forming a sliding discharge being combined with one of the electrodes of the discharge chamber, a generator having a rate of growth of output voltage of more than 1011 V/s being introduced into the device, the output of positive polarity of which is connected to the initiating electrode, while the output of negative polarity of the pulsed generator is connected to the trigger electrode of the system for forming a sliding discharge.
A dielectric insert in which an axial aperture is formed is preferably introduced into the discharge chamber, and the electrodes of the discharge chamber are disposed on the surface of the dielectric insert.
A cylindrical plasma envelope having high conductivity forms in the discharge region as a result of pre-ionization. This establishes the initiation of a pinch-type discharge under ideal conditions and ensures an increase in the output of short-wave radiation from the hot plasma discharge. In contrast to providing a substantially paraxial pre-ionization, the cross-sectional size of the pinch-type discharge is advantageously increased according to the invention when it is initiated. This makes it possible to increase the kinetic energy of the plasma substantially at the stage when it is compressed by the magnetic field of the discharge, and this ensures a more effective heating of the plasma column and an increase in the energy of the short-wave radiation, and also in its average power in the periodically pulsed mode. The use of a high rate of growth of the discharge voltage (more than 1011 V/s) establishes a highly stable initiation of a homogeneous sliding discharge that achieves pre-ionization and, in turn, ensures the possibility of achieving a high stability of the energy of the short-wave radiation from the plasma of the pinch-type discharge.